1. Field of the Invention
The present invention relates to a display integrated type position reading apparatus, and more particularly, to improvement of position reading accuracy in reading out a specified position on a thin film and displaying it on a screen in an apparatus having a transparent thin film resistor mounted integrally on a display screen such as a liquid crystal plate.
2. Description of the Background Art
FIGS. 7 and 8 show structures of a display integrated type tablet apparatus employed in a conventional apparatus and in an embodiment of the present invention. Referring to FIG. 7, a display integrated type tablet apparatus includes a CPU (Central Processing Unit) 1 for centralized management and control of the apparatus, a ROM 2 and a RAM 3 for storing programs and data, a clock 4, an I/O (Input/Output) unit 5 including a printer and a CRT (Cathode Ray Tube) connected to the apparatus, a keyboard 6 provided for entering external data, a display controller 12 controlled by the CPU 1, an A/D converter (analog/digital converting circuit) 8, a tablet controller 9, a tablet 10 controlled by the controller 9, and a display device 11 formed of a liquid crystal plate driven by the display controller 12 for display. The liquid crystal plate includes a plurality of liquid crystal elements arranged in a matrix. Each element is driven by the controller 12 for screen display.
FIG. 8 shows the structure of the tablet 10 and the display device 11. The tablet 10 is formed of a structure where an X direction transparent film resistor 10a which is a transparent thin film resistor having an electrode provided at both ends (depicted as a thick line in the drawing) of the horizontal direction in the drawing (referred to as the X direction hereinafter) and a Y direction transparent film resistor 10b which is a thin film resistor having an electrode provided at both ends (depicted as a thick line in the drawing) of the vertical direction in the drawing (referred to as the Y direction hereinafter) are overlaid with a very small uniform gap therebetween. The display screen of the display device 11 is positioned at the side of the film resistor 10b opposite to the film resistor 10a. The X direction transparent film resistor 10a has the current flow from the X.sub.H side electrode towards the X.sub.L side electrode at the time of voltage application by the controller 9. The Y direction transparent film resistor 10b has the current flow from the Y.sub.H side electrode towards the Y.sub.L side electrode at the time of voltage application by the controller 9. It is assumed that voltage is applied to film resistors 10a and 10b by the controller 9. When a user depresses the surface of the tablet 10 with the tip of a pen or a pencil, the film resistor 10a is brought into contact with the film resistor 10b at that depressed position, whereby the depressed position is detected by a voltage signal level in accordance with the divided resistance on the film resistors 10a and 10b. The voltage signal is provided from the X direction transparent film resistor 10a and the Y direction transparent film resistor 10b to be applied to the CPU 1 as XY coordinate values via the A/D converter 8. The CPU 1 drives the display controller 12 in response to the applied coordinate values, whereby the controller 12 drives the corresponding elements on the display device 11 for display. Thus, when a user depresses a desired position on the tablet 10, the depressed position can be confirmed visually by screen display.
In the above-described display integrated type tablet apparatus where the display device 11 and the tablet 10 of transparent film resistors 10a and 10b are integrated, there was a problem of lowering the accuracy of the coordinate measurement because of noise mixed into the voltage signal entering the A/D converter 8 caused by the driving signal from the controller 12 to the display device 11. That is to say, there was a problem of generation of reading out coordinates without order. Therefore, means were provided for reducing noise in the detected signal of the display integrated type tablet apparatus as shown in FIGS. 9 and 10. Referring to FIG. 9, condensers CX and CY for noise removal were inserted in the X direction and the Y direction, respectively, at the terminals for coordinate measurement, i.e. input terminals of the A/D converter 8. Referring to FIG. 10, noise is reduced in the detected signal from the tablet 10 due to driving the display device 11 by increasing the distance L.alpha.between the display device 11 and the tablet 10 (the Y direction transparent film resistor 10b).
By such means, noise overlaid in the detected coordinates, i.e. the detected voltage signal, of the tablet 10 is reduced to suppress measurement error significantly.
However the above-described means for noise reduction had the following problems.
In the case where particular elements for noise reduction such as condensers CX and CY are provided as shown in FIG. 9, it was technically difficult to completely remove noise only with these noise reduction elements. There was a problem of increase in the cost of the apparatus itself due to the number of components being increased even by the additional provision of these elements.
By increasing the distance L.alpha. between the display device 11 and the tablet 10 as shown in FIG. 10, the distance between the main surface of the tablet 10 and the screen of the display device 11 was unnecessarily increased to result in inadequate parallax. There was also a problem that the thickness of the apparatus is increased in the case of a structure where the tablet 10 and the display device 11 are formed integrally.
The display integrated type tablet apparatus as a man-machine interface is becoming increasingly important with the need of a larger screen, higher accuracy (corresponding to the critical display drive of the display device), lower power consumption, and lower cost. Although the above described method employing transparent film resistors 10a and 10b is superior from the standpoint of low cost and low power consumption, the amount of noise is increased from the display device 11 in accordance with increase in the screen size, resulting in lowering the position reading accuracy. That is to say, because the tablet 10 and the display device 11 establish an electrostatic capacitive coupling, there is a disadvantage of increase in the amount of noise included in the position read out signal in proportion to the area size of the main surface of the tablet 10.